An aggregate always operates at the level of a single transaction. What else is important to know about aggregates in DDD?

In DDD, an aggregate is a cluster of related objects that are treated as a single unit for data modification purposes. Besides operating within a single transaction, it is important to remember:

• An aggregate has a root (Aggregate Root), through which access and modification of the entire aggregate occur.
• The external world interacts only with the aggregate root, ensuring invariants and data integrity.
• Aggregates help limit transaction and consistency boundaries, reducing complexity and increasing performance.
• It is important to design aggregates so that they are not too large, otherwise issues with performance and concurrency may arise.

Example: in an order system, an order can be an aggregate, the order itself is the root, and order items are child entities accessed through the order.